Monitoring, Control and Communications for Electrical Apparatus

Discoveries from the Application of On-Line Monitoring to Substation Equipment

Contributor: Brian Sparling, SMIEEE

There are 4 major sub-systems to a transformer

> Main Tank, Including - Windings

- Oil - Core - Leads and connection

> On Load Tap Changer > Cooling System > Bushings

Global Failure rate for transformers is 1 – 2% per year

Failures are RANDOM by nature

Transformer monitoring rationale

Transformer monitoring rationale

Where does one start?

Relative importance of each component as a contributing factor

0

5

10

15

20

25

30

35

40

Core and

coils

Tap

Changer

Bushings Cooling

System

Other

Perc

enta

ge (

%)

Australia

Eskom

CEA

Location of Transformer Failure

Main tank

LTC

Bushing

Cooling system

Other

MAIN TANK: Windings failure, Partial Discharge, Excessive H2O, surface contamination,

connection failure, screens failure, circulating currents, overloading hazard,

Main Tank 30%

OLTC 35%

Bushing 15%

Cooling System 5%

Other 15%

OLTC contact overheating and coking, desynchronization, excessive number of operations,

mechanical damage, neutral switch inactivity

Bushing moisture penetration, oil leak, Partial Discharge, ‘X’ wax formation,

Cooling System clogged coolers by pollen or dust, fan/pump failure, sludge, oil leak,

Other External, such as lightning, animal interference, operation error, that are not detectable by on-line monitoring

Transformer monitoring rationale

Transformer monitoring rationale

What monitoring does one install, and on which units?

Transformer Condition Assessment

Detailed condition assessment

Weighting and Ranking

Mitigation

Refurbishment

Replacement

Health Index

Updated continuously

Criticality Index

Updated as needed

Impact of age on Health Index Large population of Power Transformer

Health Index decreases with age. HI drop is marginal.

Initial Condition Rating: Provides some clues where to start

Double Coverage

Market place

No Spare

Owner

Unit Number

Critical Index

Data Score

Power Plant

Conditio

n R

ating (

5 G

ood –

1 P

oor)

Monitoring vs. Diagnostics

No

8%

Diagnostic

2% Yes

Maintenance & Repair

Do something

else

Monitoring

10% No Is it Normal ?

Transformer

Do nothing

else

Yes

90%

Is it Serious ?

Cigre Report No. 227, Life Management Techniques for Power Transformers. WG A2.18

Broadband

technique

applied routinely Focused

technique

applied as required Maintenance &

Repair Shop

Real Life Scenario – 1/7/2010

• 7:20 AM: Receive Emergency Hot Spot Alarm at Glade Station as the Phase A Hot Spot ultimately exceeds 143 Degrees C.

• Transformer did not have hot spot winding gauge prior to Transformer Monitoring Package.

• 10:30 AM: Station inspection reveals that the Glade #1 transformer (8.4/10.5MVA – 65 OA/FA), has a single cooling fan and not the required minimum of three fans.

• 3:00 PM: 4 additional fans are added and placed on manual.

• 1/8/2010 - 7:10 AM: DDC quote to Station Management – “We checked the temperatures this morning from SCADA and the fans definitely made a difference. The winding temp is running 20 to 40 degrees C lower than the previous night with similar temperatures”

Top Oil = 74 Top Oil = 45

With Fans On

Example of a critical component

• Bushings on transformers are one of the highest failure components

• Offline testing can provide a false sense of security since many defects are:

– Temperature dependent

– Voltage dependent

– Occur very quickly

● The quality of the insulation of high voltage bushings & high voltage current transformers

● Measurements include

– Imbalance current

– Imbalance current temperature coefficient

• A very good indication of early stage insulation deficiency and contamination

– Imbalance current trend

– Calculated power factor

● Imbalance current vector position; on a Polar Plot feature in the Athena Software

Easy to interpret defective phase by Users

What is measured on-line - Bushings

Power factor change Capacitance change

Phase A

Ph

ase

A

Tanδ behaviour of a defective bushing

Power Factor as a Function of Voltage and Temperature

0

5

10

15

20

25

0 50 100 150 200 250

Hours

% P

F

10 Kv @ 25C 10 kV @ 70C 70 kV @ 25C 70 kV @ 70C

1

2

3

4

An example of lesson learned

Bushing Monitor Unit installed in June 2005

Transformer failed in August 25, 2005 at 5:30 PM

Customer did not have relay alarm contacts or communications wired to control room

Monitor was alarming, but no one knew

Result, 150 MVA actual failure

A-Phase

Components Of an Intelligent Condition Monitoring System

Information Visualization

Data Processing

Data Exchange

Data management

Data Acquisition

Data Measurement

• Do not throw it away!

• No need to look at ‘data’ every day or hour.

• Have a response plan in place to respond to alarms (add to existing alarm management strategy)

• Use the data to understand what is happening and why.

• What you will have is a record for the “behaviour’ of the transformer under operating conditions!

• Now one could use advanced statistical analysis to understand normal behaviour of the transformer

• Using the same analysis be able to spot when ‘abnormal’ behaviour begins

• Using this method has revealed problems BEFORE, alarms from a monitor

Now I have data, and lots of it, now what?

Statistical model of gassing behaviour

Single gas monitor on a transformer Normal behaviour for many weeks before it changes

Behaviour changes 5.2 days before the transformer failed. Operator did not respond to the alarms from the monitor (red line)

Statistical Model of behaviour of all data

Components Of an Intelligent Diagnostic System

• On-Line monitoring is an enabler for understanding transformer behaviour in operation.

• Provides for early detection of incipient failure conditions that time based testing methods can miss.

• Provides the opportunity to move to Condition Based Maintenance (CBM) from time based methods.

• Now have the data to use statistical methods to make use of all the accumulated data from monitoring systems

• Serves many masters;

Asset Managers

Operations and maintenance engineers

Systems Operators

Life extension of existing assets.

Summary

• Once the decision is made to go with monitoring;

– Develop a change management process

– Obtain ‘buy in’ from others who will be affected (IT etc.)

– Implement new and/or revise the alarm response

procedures for a net set of more specific alarms

– Do not forget, or delay communications connections, and

path for alarms.

What is still Missing?

• The following guides will assist those interested in further study of monitoring of transformers and how to utilize the results;

• 1) IEEE C57.143-2012, Guide for Application for Monitoring Equipment to Liquid-Immersed Transformers and Components

• 2) C57.140-2006, Guide for the Evaluation and Reconditioning of Liquid Immersed Power Transformers

• 3) C37.10.1-2000, Guide for The Selection of Monitoring for Circuit Breakers

• 4) CIGRE TB 445, April 2008, Guide for Transformer Maintenance

Further Reading and Study

Brian Sparling, SMIEEE Dynamic Ratings

IEEE T&D Expo, Chicago IL

April 2014

25

26